1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases. More particularly, it relates to the catalyst which can efficiently purify nitrogen oxides (NO.sub.x) in the exhaust gases whose oxygen concentrations are at the stoichiometric point or more than required for oxidizing carbon monoxide (CO) and hydrocarbons (HC) therein.
2. Description of Related Art
As catalysts for purifying automotive exhaust gases, there have been employed 3-way catalysts so far which oxidize CO and HC and reduce NO.sub.x to purify the exhaust gases. For example, the 3-way catalysts have been known widely which comprise a heat resistant honeycomb-shaped monolithic support formed of cordierite and having cellular walls, a porous layer formed of gamma-alumina and disposed on the cellular walls, and a noble metal catalyst ingredient selected from the group consisting of Pt, Pd and Rh and loaded on the porous layer.
The purifying performance of the 3-way catalysts for purifying exhaust gases depends greatly on the air-fuel ratio A/F of automotive engine. For instance, when the air-fuel weight ratio is larger than 14.6, i.e., when the fuel concentration is low (or on the fuel-lean side), the oxygen concentration is high in exhaust gases (hereinafter simply referred to as "fuel-lean atmospheres"). Accordingly, the oxidation reactions purifying CO and HC are active, but the reduction reactions purifying NO.sub.x are inactive. On the other hand, when the air-fuel ratio is smaller than 14.6, i.e., when the fuel concentration is high (or on the fuel-rich side), the oxygen concentration is low in exhaust gases (hereinafter simply referred to as "fuel-rich atmospheres"). Accordingly, the oxidation reactions are inactive, but the reduction reactions are active.
Moreover, when driving automobiles, especially when driving automobiles in urban areas, the automobiles are accelerated and decelerated frequently. Consequently, the air-fuel ratio varies frequently in the range of from the values adjacent to the stoichiometric point (or the theoretical air-fuel ratio: 14.6) to the fuel-rich side. In order to satisfy the low fuel consumption requirement during the driving conditions such as in the above-described urban areas, it is necessary to operate the automobiles on the fuel-lean side where the air-fuel mixture containing oxygen as excessive as possible is supplied to the engines. Hence, it has been desired to develop a catalyst which is capable of adequately purifying NO.sub.x even in exhaust gases of fuel-lean atmospheres.
In view of the aforementioned circumstances, the applicants et al. of the present invention proposed a novel catalyst in Japanese Unexamined Patent Publication (KOKAI) No. 5-317,652. In this catalyst, an alkaline-earth metal and Pt are loaded on a support including porous substance.
In accordance with the novel catalyst, in exhaust gases of fuel-lean atmospheres, NO.sub.x, which includes NO in an amount of about 90% by volume and the balance of NO.sub.2 etc., is stored in the alkaline-earth metal elements. In particular, the NO is oxidized to NO.sub.2 by the Pt. The resulting NO.sub.2 is reacted with the alkaline-earth metal elements to produce alkaline-earth metal nitrates (e.g., barium nitrate, Ba(NO.sub.3).sub.2), thereby being stored in the alkaline-earth metal elements. When the air-fuel mixture varies from the stoichiometric point to the fuel-rich atmospheres, the stored NO.sub.2 is released from the alkaline-earth metal elements, and it is reacted with HC, CO and the like, included in exhaust gases, by the action of the Pt. Thus, NO.sub.x is reduced and purified to N.sub.2. As a result, the catalyst exhibits superb NO.sub.x purifying performance in fuel-lean atmospheres.
In other words, NO components are present in a large amount in NO.sub.x included in exhaust gases, but they cannot be stored directly on the NO.sub.x storage component (e.g., an alkaline-earth metal, etc.). That is, after NO components are oxidized to NO.sub.2 by the oxidation action of the noble metal catalyst ingredient (e.g., Pt, etc.), they are stored in the NO.sub.x storage component at last. Namely, the NO.sub.x storage component cannot store NO.sub.x therein by itself, and it can maximumly exhibit its NO.sub.x storing capability when it is disposed adjacent to the noble metal catalyst ingredient, such as Pt and the like.
When producing the above-described novel exhaust-gases-purifying catalyst, the noble metal catalyst ingredient, for example Pt, is loaded as follows: a honeycomb-shaped monolithic support is prepared which has cellular walls, and a porous layer formed of alumina or the like and disposed on the cellular walls. Then, the support is immersed into a platinum dinitrodiammine aqueous solution of low concentration. After a predetermined time has passed, the support is taken out of the aqueous solution, dried and calcinated.
Accordingly, the platinum dinitrodiammine aqueous solution is impregnated into the porous layer from the outer portion to the inner portion in this order, and at the same time it is impregnated into pores of the porous substance (e.g., alumina) in the outer and inner portions. The terms, "outer portion" and "inner portion," herein have the following meaning: when the porous layer coated on cellular walls of the support is viewed cross-sectionally, the "outer portion" denotes a part of the porous layer which contacts with exhaust gases flowing through the support, and the "inner portion" denotes a part of the porous layer which contacts with the support.
In the aforementioned Pt loading, the platinum dinitrodiammine aqueous solution of low concentration is prepared in a volume equal to or more than a water storing capability exhibited by the support. When the porous layer is constituted by alumina, the term, "water storing capability," herein means a total amount of the aqueous solution which can be filled in pores of the alumina itself. When Pt is loaded on the support in an amount of about 1 gram with respect to 1 liter of the support, and when a water storing capability of the support is about 0.2 liters with respect to 1 liter of the support, the aqueous solution contains Pt in an amount of about 5 grams with respect to 1 liter of the aqueous solution.
Thus, in the Pt loading described above, the Pt content is low and the aqueous solution is prepared in a large volume, Pt is included in low concentration in the aqueous solution, and it is very likely to be stored in the porous support formed of alumina, etc. Accordingly, a major portion of Pt is instantaneously stored and loaded on the outer portion, and substantially no Pt is present to be loaded on the inner portion. As a result, Pt is loaded in a distribution in which Pt is loaded more on the outer portion but less on the inner portion.
On the other hand, the alkaline-earth metal, for example Ba, is loaded as follows: the support with Pt loaded is immersed into an alkaline-earth metal compound (e.g., barium acetate) aqueous solution of high concentration. Then, the support is dried and calcinated while it holds the aqueous solution fully therein (e.g., in all of the pores of the alumina itself). In this type of alkaline-earth metal loading, the alkaline-earth metal is loaded virtually uniformly from the outer portion to the inner portion of the porous layer.
In the alkaline-earth metal loading, the barium acetate aqueous solution of high concentration is prepared in a volume substantially equal to the water storing capability exhibited by the support. When Ba is loaded on the support in an amount of about 0.2 moles with respect to 1 liter of the support, and when the support exhibited the same water storing capability as above, the aqueous solution contains barium being in the metallic form in an amount of about 137 grams (1 mole) with respect to 1 liter of the aqueous solution.
The Pt loading and the alkaline-earth metal loading are thus different from each other. The difference results in that Pt is loaded in an amount decreasing from the outer portion to the inner portion in the porous layer, and that the alkaline-earth metal is loaded uniformly from the outer portion to the inner portion in the porous layer.
All in all, in the outer portion of the porous layer, Pt and the alkaline-earth metal meet each other with high probability. On the contrary, in the inner portion of the porous layer, Pt and the alkaline-earth metal meet each other with low probability, and accordingly Pt is not present adjacent to the alkaline-earth element. As a result, the alkaline-earth metal loaded on the inner portion might not be able to exhibit its function at all. Namely, it might not store NO.sub.x thereon in exhaust gases of fuel-lean atmospheres. Specifically, since it might not store NO.sub.x thereon and it might not release NO.sub.x either, no reductive purifying action might arise. Thus, the novel exhaust-gases-purifying catalyst described above might not be improved in term of NO.sub.x purifying performance.
In order to solve the aforementioned problems, it is possible to increase the loading amount of Pt or to concentrate the concentration of Pt by reducing the volume of the water of the platinum dinitrodiammine aqueous solution. However, these countermeasures cannot be taken because of the following reasons. Namely, the increment in the Pt loading amount pushes up production cost inevitably. Since the volume reduction in the water does not result in the Pt content variation, and since Pt is likely to store in the porous layer invariably, Pt is still loaded on the outer portion of the porous layer in a large amount.